Printing devices are known which employ a drum to whose outer surface is secured a flexible data carrier on which an image is to be written. Thus, for example, photo plotters may include a drum having attached thereon a photographic film which is exposed to a light source as the drum rotates. In high speed laser plotters, for example, the speed of rotation of the drum can often be sufficiently high that there exists a tendency for the film to be lifted from the drum at one or both of the leading and trailing edges of the film. The forces which tend to cause the film to lift from the surface of the drum include centrifugal and aerodynamic lift forces as well as natural bending forces within the film itself.
As soon as an edge of the film starts to lift from the surface of the drum, there is created a pocket into which air can flow, either directly at a leading edge or on account of turbulence at a trailing edge. In either case, the inflow of air tends to lift the film even further from the surface of the drum.
Typically, photographic films and the like are secured to the rotating drum by means of suction applied through apertures provided in the shell of a hollow drum. In order to overcome, or at least minimize, the effect of edge lift of the film, it is known to employ mechanical fixing means at each edge of the film. However, this increases the set-up time as well as the time for removing the film from the drum after exposure. Another approach is simply to increase the strength of the vacuum producing the suction. However, this requires correspondingly larger compressors and clearly increases the cost of the resulting system. Furthermore, increasing the vacuum in this manner, can result in the film being partially sucked through the drum apertures, thus causing distortion of the film where it overlies the apertures. Such distortion causes the image at these points to be defocused and is clearly undesirable.
Although the lift effect described above is clearly exacerbated as the speed of rotation is increased, it exists even when the drum is stationary if the film is secured thereto by suction. This is because any natural tendency which the film may have to lift from the drum at its edges permits air to enter, thereby reducing the strength of the suction at the edges and increasing the lift effect.